High reliability ac load switching circuit

ABSTRACT

A high reliability AC load switching circuit is disclosed. In some embodiments, the AC load switching circuit includes a high-speed switch connected between the load and the voltage source, a cutoff switch connected between the load and the voltage source in parallel with the high-speed switch, and a level detector connected to the voltage source and to a control input of the high-speed switch. The high-speed switch may be a solid-state switch, for example, a TRIAC or a bidirectional switch, and the cutoff switch may be an electromechanical switch, for example, a relay. In some embodiments a snubber is connected in parallel with a solid-state switch. In some embodiments a microcontroller is connected to an eletromechanical switch and the level detector. In some embodiments, both a first cutoff switch and a second cutoff switch are used.

BACKGROUND

Plug-in loads are now the third highest contributor to electricity usagenext to lighting and HVAC (heating, ventilation and air conditioning) inmost office buildings, and this situation is expected to increase asmore occupants use individual devices such as computers, displays, andprinters. Some commercial building electrical systems are being designedto automatically reduce power use in unoccupied spaces through automaticreceptacle control for at least some of the receptacles in the building.

Receptacles in such building are normally powered via a switchingcircuit. When the switching circuit is turned on, electric current flowsto the load. Often, a large in-rush current occurs, which can trip acircuit breaker if the switching circuit does not have proper control ofthe turn-on moment for capacitive loads. In switching circuits based onsolid-state devices, excessive power loss and heat may result if theload draws high current, leading to failure of the component if coolingis inadequate. Some semiconductor devices can become leaky and allowcurrent to flow to the load even when shut off, depending on temperatureand other factors, which can cause abnormal operation of the load andwasted power.

SUMMARY

Embodiments of the present invention include a high-speed switch orswitching device such as a triode for alternating current (TRIAC) or abidirectional solid-state switch, at least one cutoff switch, and azero-crossing detection and control circuit, which may be referred toherein as a “level detector.” The cutoff switch may be or include anelectromechanical switch, as an example, a relay.

In some embodiments, a switching circuit to selectively connect a loadto a voltage source includes a high-speed switch connected between theload and the voltage source, a cutoff switch connected between the loadand the voltage source in parallel with the high-speed switch, and alevel detector connected to the voltage source and to a control input ofthe high-speed switch. In some embodiments a snubber is connected inparallel with the high-speed switch. In some embodiments amicrocontroller is connected to the cutoff switch and the leveldetector.

Some of the above embodiments include at least a first cutoff switch anda second cutoff switch, with the second cutoff switch being connected inseries with the high-speed switch and the first cutoff switch. Thesecond cutoff switch can be connected either between the high-speedswitch and the voltage source or between the high-speed switch and theload.

In any of the above embodiments, the high-speed switch can be or includea solid-state switch. In any of these embodiments, the solid-stateswitch can be or can include a triode for alternating current (TRIAC) ora bidirectional switch. If the cutoff switch is implemented with anelectromechanical switch such as a relay, and a microcontroller is usedto control the switching circuit, a transistor can be connected to themicrocontroller, and a resistor can then be connected between thetransistor and the electromechanical switch to allow the microcontrollerto control the electromechanical switch. This configuration can be usedfor a single eletromechanical switch or each of multipleelectromechanical switches.

The switching circuit can be operated according to example embodimentsof the invention by firmware or control circuitry. The switching circuitis operated by turning on a solid-state switch between the voltagesource and the load at zero-crossing of the AC voltage, and activating afirst electromechanical switch connected in parallel with the solidstate switch after the turning on of the solid-state switch. When thecircuit is turned off or powered down, the first electromechanicalswitch is deactivated and then the solid-state switch is turned off.

If a second electromechanical switch is connected in series with thesolid-state switch it is turned on prior to turning on the solid-stateswitch in response to activation of the switching circuit. Duringpower-down, the first electromechanical switch is deactivated, followedby the turning off of the solid-state switch and the deactivation ofsecond electromechanical switch.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating a switching circuit accordingto some embodiments of the present invention.

FIG. 2 is a schematic diagram illustrating a switching circuit accordingto additional embodiments of the present invention.

FIG. 3 is a schematic diagram illustrating a switching circuit accordingto further embodiments of the present invention.

FIG. 4 is a schematic diagram illustrating a switching circuit accordingto more example embodiments of the present invention.

FIG. 5 is a schematic diagram illustrating additional embodiments of theinvention where a bidirectional transistor switch is used.

FIG. 6 is a flowchart illustrating the operating process of a switchingcircuit according to embodiments of the invention.

FIG. 7 is a flowchart illustrating one of the subprocesses of theoperating process of FIG. 5.

DETAILED DESCRIPTION

Embodiments of the present invention now will be described more fullyhereinafter with reference to the accompanying drawings, in whichembodiments of the invention are shown. This invention may, however, beembodied in many different forms and should not be construed as limitedto the embodiments set forth herein. Rather, these embodiments areprovided so that this disclosure will be thorough and complete, and willfully convey the scope of the invention to those skilled in the art.Like numbers refer to like elements throughout.

It will be understood that, although the terms first, second, etc. maybe used herein to describe various elements, these elements should notbe limited by these terms. These terms are only used to distinguish oneelement from another. For example, a first element could be termed asecond element, and, similarly, a second element could be termed a firstelement, without departing from the scope of the present invention. Asused herein, the term “and/or” includes any and all combinations of oneor more of the associated listed items.

It will be understood that when an element such as a layer, region orsubstrate is referred to as being “on” or extending “onto” anotherelement, it can be directly on or extend directly onto the other elementor intervening elements may also be present. In contrast, when anelement is referred to as being “directly on” or extending “directlyonto” another element, there are no intervening elements present. Itwill also be understood that when an element is referred to as being“connected” or “coupled” to another element, it can be directlyconnected or coupled to the other element or intervening elements may bepresent. In contrast, when an element is referred to as being “directlyconnected” or “directly coupled” to another element, there are nointervening elements present.

Relative terms such as “below” or “above” or “upper” or “lower” or“horizontal” or “vertical” may be used herein to describe a relationshipof one element, layer or region to another element, layer or region asillustrated in the figures. It will be understood that these terms areintended to encompass different orientations of the device in additionto the orientation depicted in the figures.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”“comprising,” “includes” and/or “including” when used herein, specifythe presence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof.

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which this invention belongs. It will befurther understood that terms used herein should be interpreted ashaving a meaning that is consistent with their meaning in the context ofthis specification and the relevant art and will not be interpreted inan idealized or overly formal sense unless expressly so defined herein.

Unless otherwise expressly stated, comparative, quantitative terms suchas “less” and “greater”, are intended to encompass the concept ofequality. As an example, “less” can mean not only “less” in thestrictest mathematical sense, but also, “less than or equal to.”

A high-reliability AC load switch according to embodiments of theinvention is provided by a switching circuit, which might also be termeda power control module. The power control module or switching circuitincludes circuit configurations and a control process.

FIG. 1 is a schematic diagram showing an example embodiment of thepresent invention. The switching circuit 100 selectively connectsvoltage source 102 to load 104. The switching circuit includes asemiconductor device that acts as a high-speed switch, in this example,a TRIAC Q1, a cutoff switch, which in this example is relay 106, and azero-crossing detection and control circuit, level detector 108. Acommercially available integrated zero-crossing detection and controlcircuit, such as part number MOC3043 from Fairchild Semiconductor, canbe used as a level detector with embodiments of the invention. Such adevice acts as a latching, optically coupled TRIAC driver by trackingthe AC voltage waveform and detecting its instantaneous level, therebydetermining when that level is zero volts because of a “zero-crossing”of the voltage waveform. Resistors R1 and R2 are used to provide thedesired driving current for the TRIAC. The solid-state semiconductordevice (TRIAC) Q1 is connected in parallel with the electro-mechanicalswitch 106.

Still referring to FIG. 1, microcontroller unit (MCU) 110 is connectedto enabling input number one (EN1) to enable level detector 108 to turnon the TRIAC at the zero-crossing of the AC voltage that occurs nextafter the enabling input EN1 becomes active. Thus, when the load (suchas a printer, a computer, a lighting fixture, etc.) needs to be poweredon, the solid-state switch is turned on only at zero-crossing of theapplied source voltage and with the enabling control signal EN1 applied.The level of an enabling control signal can be HIGH or LOW depending onthe type of microcontroller used and how the circuit is connected. Forpurposes of this disclosure, it can be assumed that a level of HIGH isused to enable or turn on a component while a level of LOW is used todisable or turn off a component. Since the input voltage is applied tothe load(s) at zero-crossing, the in-rush current due to any capacitiveload, such as the EM1 filtering capacitors or the bulk capacitors forsmoothing voltage in a personal electronic device, is significantlyreduced compared to turning on the power at moments other than that ofzero-crossing. After the solid-state switch is turned on, theelectromechanical switch 106 is turned on with enabling control signaltwo (EN2). As the voltage across the solid-state switch is generallylow, mostly around 1 V, arcing or welding (tack welding) of the contactsof the relay doesn't occur even if bouncing happens when theelectromechanical switch is being closed.

When the electrical/electronic equipment needs to be powered off, theelectromechanical switch 106 is turned off by the MCU setting controlsignal EN2 LOW while the solid-state switch is still on with EN1 beingmaintained HIGH. After the electromechanical switch 106 is turned off,the solid-state switch Q1 is then turned off with the control voltageEN1 being changed to LOW. With the two components connected in parallelturned off, there is only a small leakage current flowing to the load(s)and for many practical applications, the load(s) are considered poweredoff

Continuing with FIG. 1, it should also be noted that for the MCU toproperly drive a relay, it may need to be connected to a transistor suchas transistor Q2 as well as a current limited resistor such as resistorRL. Many types of controllers can be used, including digital signalprocessors and microprocessors of various types. Dedicated circuitry oran application specific integrated circuit (ASIC) can also be used as anMCU to provide the turn-on/turn-off signals for the enabling inputs EN1and EN2. An MCU may or may not be able to drive a relay directly,depending on the outputs provided. The relay may be referred to as anelectromechanical switch, or as a cutoff switch, since the purpose ofusing such a device is to completely cut off current flow with noleakage.

FIG. 2 is a schematic diagram of another embodiment of the presentinvention. Switching circuit 200 of FIG. 2 selectively connects voltagesource 202 to load 204. The circuit of FIG. 2 uses the same TRIAC Q1,relay 106 and resistors R1 and R2, but includes a snubber 220 comprisingresistor R3 and capacitor Cl, connected in parallel with TRIAC Q1 tosuppress voltage spikes and dampen ringing that might be caused bycircuit inductance or an inductive load. Such ringing could cause themisfiring of the TRIAC when the TRIAC needs to be turned off. Amisfiring of the TRIAC could turn on the power when not desired. Theelimination of ringing from any of these causes can result in aswitching circuit with higher reliability and lower electromagneticinterference (EMI). It can be assumed that the same type ofmicrocontroller unit (MCU) is connected to the same enabling inputs EN1and EN2 as already discussed. The MCU and supporting circuitry fordriving relay 106 are omitted in FIG. 2 for simplicity and clarity ofillustration.

FIG. 3 shows a schematic diagram of an additional embodiment of thepresent invention with a second cutoff switch connected in series withthe solid-state switch. Such an implementation works well where theleakage current must be further minimized, such as with small deviceswhere the leakage current would be a significant percentage of theoperating current. Such a device might be, as an example, an LEDlighting system such as a lamp, down-light, emergency light, or thelike. The purpose of using a second cut-off switch is to completely turnoff the power since a semiconductor switch such as the TRIAC may allowleakage current depending on the operating temperature. Circuit 300 ofFIG. 3 selectively connects voltage source 302 to LED lamp 304. Thecircuit of FIG. 3 uses the same TRIAC Q1, the same relay 106 as a firstcutoff switch, resistors R1 and R2, as well as snubber 220 comprisingresistor R3 and capacitor Cl. However, circuit 300 of FIG. 3 includes asecond cutoff switch, relay 306 connected in series with TRIAC Q1. Thesecond relay 306 in this example is connected between the voltage sourceand the solid-state switch, and is enabled by enabling input three(EN3), which is in turn connected to the microcontroller unit (MCU, notshown).

With the circuit of FIG. 3, when electrical/electronic equipment (suchas a printer, a computer or a lighting fixture etc.) needs to be poweredon, the second electromechanical switch 306 is turned on first bycontrol signal EN3, then the solid-state switch Q1 is turned on atzero-crossing of the applied source voltage after being enabled bycontrol signal EN1, and finally the first electromechanical switch 106is turned on by control signal EN2. As before, since the input voltageis applied to the load(s) at zero-crossing, the in-rush current due toany capacitive load, such as the EMI filtering capacitors or the bulkcapacitors for smoothing voltage, is significantly reduced compared toturning on the power at moments other than zero-crossing.

When the electrical/electronic equipment needs to be powered off, thefirst electromechanical switch 106 is turned off by control signal EN2,then the solid-state switch Q1 is turned off by control signal EN1, andfinally, the second electromechanical switch 306 is turned off bychanging the level of control signal EN3 from HIGH to LOW. In this “off”configuration, no current flows through the load(s) as secondelectromechanical switch 306 completely opens the circuit so that therecan be no leakage current. This implementation works for any sizeloads(s)—small, medium or large.

FIG. 4 is a schematic diagram of another embodiment of the presentinvention. Circuit 400 of FIG. 4 selectively connects voltage source 402to LED lamp 404. All of the same components are used as in circuit 300of FIG. 3. However, second cutoff switch (relay) 306 is in this caseconnected between the solid-state switch Q1 and the load. It should benoted that the second relay can also be said to be connected in serieswith the first relay (first cutoff switch) in either of the embodimentswith two electromechanical switches described above. Also, snubber 220can be omitted from a circuit with two electromechanical switches, sothat the switching circuit is similar to that shown in FIG. 1 exceptwith a second electromechanical switch. The snubber can be omitted whilestill minimizing spikes and ringing if a TRIAC with built-in ringingsuppression is used. Such so-called “snubberless” TRIACs include partnumbers T2035H-6T and BTA24-600CWRG from ST Microelectronics.

FIG. 5 is a schematic diagram of another embodiment of the presentinvention. Switching circuit 500 of FIG. 5 selectively connects voltagesource 502 to load 504. The circuit of FIG. 5 uses the same relay 106and resistor R1 but for a high-speed switch uses a bidirectional switchbased on transistors 505 and 507. Transistor 505 is an n-MOSFET andtransistor 507 is a p-MOSFET. It can be assumed that the same type ofmicrocontroller unit (MCU) is connected to the same enabling inputs EN1and EN2 as already discussed and can be programmed to control switchingcircuit 500. The MCU and supporting circuitry for driving relay 106 areomitted in FIG. 5 for simplicity and clarity of illustration.

Still referring to FIG. 5, level detector 508 determines zero crossingof the AC voltage waveform as before, and uses this information toactivate the relay and the bidirectional switch. However, supportingcircuitry 520 is used to drive the bidirectional switch. As an example,this supporting circuitry can include a transformer to provide anisolated gate-control signal to turn on or turn off transistors 505 and507 simultaneously. A pulse signal source with enough driving capabilityto provide an appropriate pulse signal is connected to the primary sideof the transformer. Integrated circuits designed for the purpose ofproviding a driving pulse signal to a transformer can be obtained, or adiscrete circuit for this purpose can be built. The same microcontrollerthat is used for the other control functions of the switching circuitmay instead be able to provide this pulse signal if the appropriate typeand model of microcontroller is selected. Alternatively, a secondmicrocontroller can instead be provided.

FIG. 6 and FIG. 7 are flowcharts that together illustrate the operationof the switching circuit according to example embodiments of theinvention. Like most software flowcharts, FIG. 6 and FIG. 7 illustratethe process as a series of process blocks. In some embodiments, ageneral-purpose processor such as a digital signal processor,microcontroller, microcontroller unit (MCU) or microprocessor is usedand non-transitory firmware, software, or microcode can be stored in atangible storage medium that is associated with the device. Such amedium may be a memory integrated into the processor, or may be a memorychip that is addressed by the processor to perform control functions.Such firmware, software or microcode is executable by the processor andwhen executed, causes the microcontroller unit to perform its controlfunctions. Such firmware or software could also be stored in or on atangible medium such as an optical disk or traditional removable orfixed magnetic medium such as a disk drive used to load the firmware orsoftware into a switching system for maintenance, update, manufacturing,or other purposes.

Process 600 of FIG. 6 begins at block 602. At block 604, the switchingcircuit is powered on. This can be as a result of building automationand a specific time of day being reached, or a person who is a user ofthe system could activate a power strip, circuit breaker, or similardevice. At block 605, the series-connected electromechanical switch isactivated if the series-connected switch is present. At block 608, thezero-crossing detector is enabled and waits for a zero-crossing of thesupply voltage. When zero-crossing occurs at block 610, the solid-stateswitch is turned on at block 612. The parallel-connectedelectromechanical switch (the “first” electromechanical switch in thecircuit descriptions) is turned on at block 614. The circuit remains on,in normal use at 616. If the circuit is powered off by either automationor a user at block 616, the power-off sequence is executed at subprocessblock 618 and the process ends at block 620.

FIG. 7 illustrates the power-down process for the switching circuitaccording to example embodiments of the invention. Subprocess 618 beginsat block 702. At block 704, the parallel-connected (“first”)electromechanical switch is deactivated. At block 706, the solid-stateswitch is turned off. At block 708, if present, the series connected(“second”) electromechanical switch is deactivated. Subprocess 618 endsat block 710.

Although specific embodiments have been illustrated and describedherein, those of ordinary skill in the art appreciate that anyarrangement which is calculated to achieve the same purpose may besubstituted for the specific embodiments shown and that the inventionhas other applications in other environments. This application isintended to cover any adaptations or variations of the presentinvention. The following claims are in no way intended to limit thescope of the invention to the specific embodiments described herein.

1. A switching circuit to selectively connect a load to a voltagesource, the switching circuit comprising: a high-speed switch connectedbetween the load and the voltage source; a cutoff switch connectedbetween the load and the voltage source in parallel with the high-speedswitch; and a level detector connected to the voltage source and to acontrol input of the high-speed switch.
 2. The switching circuit ofclaim 1 further comprising a snubber connected in parallel with thehigh-speed switch.
 3. The switching circuit of claim 1 furthercomprising a microcontroller connected to the cutoff switch and thelevel detector.
 4. The switching circuit of claim 3 wherein thehigh-speed switch comprises solid-state switch.
 5. The switching circuitof claim 4 wherein the solid-state switch comprises a triode foralternating current (TRIAC).
 6. The switching circuit of claim 4 whereinthe solid-state switch comprises bidirectional switch.
 7. The switchingcircuit of claim 3 further comprising a snubber connected in parallelwith the high-speed switch.
 8. The switching circuit of claim 3 whereinthe cutoff switch comprises an electromechanical switch and furthercomprising: a transistor connected to the microcontroller; and aresistor connected between the transistor and the electromechanicalswitch.
 9. The switching circuit of claim 8 wherein the high-speedswitch comprises a solid-state switch.
 10. The switching circuit ofclaim 9 wherein the solid-state switch comprises a triode foralternating current (TRIAC).
 11. The switching circuit of claim 9wherein the solid-state switch comprises bidirectional switch.
 12. Theswitching circuit of claim 8 further comprising a snubber connected inparallel with the high-speed switch.
 13. A switching circuit toselectively connect a load to a voltage source, the switching circuitcomprising: a high-speed switch connected between the load and thevoltage source; a first cutoff switch connected between the load and thevoltage source in parallel with the high-speed switch; a second cutoffswitch connected in series with the high-speed switch; and a leveldetector connected to the voltage source and to a control input of thehigh-speed switch.
 14. The switching circuit of claim 13 furthercomprising a snubber connected in parallel with the high-speed switch.15. The switching circuit of claim 13 further comprising amicrocontroller connected to the first cutoff switch, the second cutoffswitch, and the level detector.
 16. The switching circuit of claim 15wherein the high-speed switch comprises a solid-state switch.
 17. Theswitching circuit of claim 16 wherein the first cutoff switch, thesecond cutoff switch, or both cutoff switches comprise anelectromechanical switch.
 18. The switching circuit of claim 17 whereinthe electromechanical switch comprises a relay.
 19. The switchingcircuit of claim 16 further comprising a snubber connected in parallelwith the solid-state switch.
 20. The switching circuit of claim 16wherein the second cutoff switch is connected between the solid-stateswitch and the voltage source.
 21. The switching circuit of claim 16wherein the second cutoff switch is connected between the solid-stateswitch and the load.
 22. A method of selectively connecting a load to anAC voltage through a switching circuit, the method comprising: turningon a solid-state switch between the voltage source and the load atzero-crossing of the AC voltage; and activating a firstelectromechanical switch connected in parallel with the solid-stateswitch after the turning on of the solid-state switch.
 23. The method ofclaim 22 further comprising: deactivating the first electromechanicalswitch; and turning off the solid-state switch.
 24. The method of claim22 further comprising activating a second electromechanical switchconnected in series with the solid-state switch prior to turning on thesolid-state switch and in response to activation of the switchingcircuit.
 25. The method of claim 24 further comprising: deactivating thefirst electromechanical switch; turning off the solid-state switch; anddeactivating the second electromechanical switch.